Bug#992673: libgpuarray: *gemv broken on libclblast

[Rebecca N. Palmer]

Control: retitle -1 libgpuarray: *gemv broken on libclblast

For now I have disabled *gemv when used with libclblast, to make this 
bug an explicit error rather than a silently wrong answer.

These functions are still available with clblas, but that has enough 
other issues (https://bugs.debian.org/cgi-bin/pkgreport.cgi?src=clblas) 
that it has been removed from Debian testing.

(The autopkgtest failures on armel/armhf look like a separate bug, 
possibly in pocl rather than here.)